Probably you should implement a method that listen to text change in UITextField. using
[textField addTarget:self action:@selector(textFieldDidChange:) forControlEvents:UIControlEventEditingChanged];
So that you should be knowing whenever user enter a charachter in UITextFiled from that you can check if user has entered white space delete that and prompt user to write again.
Here is how you can delete last charecter that user have entered whenever textFieldDidChange: is called and you checked that user has entered white space
if ( [myString length] > 0)
{
     myString = [myString substringToIndex:[string length] - 1];
}